Wired through a connector to corresponding lights on the towing vehicle, trailers lights typically experience problems at this connection point. Connectors vary and can range from four to seven terminals. The typical four terminal connector used on small trailers provides tail, left turn and brake, right turn and brake and a ground return. Larger connectors can add electric brakes, reversing lights and 12 volts for accessories. Most wiring problems occur at the connectors or where the wire connects to the light. Other areas to watch are where the wiring flexes, such as on a trailer with a tilting bed.
Disconnect the trailer connector and check the continuity between the ground terminal on the vehicle connector and the vehicle chassis using the volt ohm meter. Typically the ground wire is the white wire on the connector and is often connected to a reversed direction terminal on the connector. If there is no continuity, replace the wire between vehicle chassis ground and the connector.
Check continuity between the ground terminal on the trailer connector and the trailer chassis. As in step one, the ground wire is usually the white wire on the connector and it is often connected to a reversed direction terminal on the connector. If there is no continuity, replace the wire between the trailer chassis and the connector.
Clean the exposed terminals on the vehicle and trailer connectors using fine sandpaper or a pencil eraser. Be sure to wipe the terminals to remove any debris after cleaning. Restoring the ground connection should fix the situation if all lights are not working.
Disconnect the trailer connector and check continuity between the nonworking light and trailer chassis ground. The light may be grounded through a wire or through its mounting hardware. If so, make sure there is no dirt or paint under the mounting hardware. The light's ground also may be hard-wired to the ground on the trailer connector. If there is no continuity, repair the light's ground connection.
Turn on the vehicle light corresponding to the nonworking light on the trailer. Check the vehicle connector for 12 volts on the terminal for that light. Typical wire colours are right turn and brake: green; left turn and brake: yellow; tail and markers: brown; backup: purple. If 12 volts are not present, troubleshoot the vehicle wiring.
Check continuity between the trailer connector and the nonworking light. Trailer wiring is typically the same colours as noted in Step 2. If there is no continuity, replace the wire between the nonworking light and the trailer connector.
Many trailer lights are grounded through the mounting hardware. If the light is mounted to plastic instead of the metal trailer chassis, it will not work and a ground wire must be added. Be sure to check vehicle fuses and for possible trailer mounted fuses.
Do not work on the wiring while power is applied to the circuit. A 12 volt spark can ignite flammable fumes or liquids on the vehicle or trailer.